Lithography is at the heart of processes for the fabrication of microelectronic devices, such as integrated circuits, and micromechanical structures. The basic process of producing a microelectronic device involves the modification of the surface material of a semiconductor substrate, such as of silicon, in a pattern. The interplay of the material changes and the pattern defines the electrical characteristics of the microelectronic device. A similar process can be used to form micromechanical devices, by, for example, electroplating metal structures in a desired pattern onto a substrate. Lithography is used to define the pattern on the substrate which will be doped, etched, or otherwise modified to form the microelectrical or micromechanical device.
In a basic lithography process for the fabrication of microelectronic or micromechanical devices, a photo sensitive material, such as polymethylmethacrylate (PMMA), is deposited on a substrate surface. The photoresist is sensitive to radiation, e.g., X-rays or extreme ultraviolet (EUV) radiation, and, depending on the photoresist used, portions of the photoresist that are exposed to the radiation may be removed (or left remaining) by a development process. The microelectronic or micromechanical device is formed by etching or otherwise modifying the substrate in the areas from which the photoresist has been removed. To form a desired pattern in the photoresist, the radiation that is used to expose the photoresist is passed through or reflected off of a lithography mask that defines the pattern that is to be transferred to the photoresist.
An exemplary portion of an EUV lithography mask 10, which may be used in a microelectronic device fabrication process, is illustrated schematically in FIG. 1. The EUV mask 10 is formed on an EUV mask blank 12 which includes an EUV reflective substrate 14 upon which are deposited multiple layers of material forming an interference stack 16 which enhances the overall EUV reflectivity of the mask 10. In general, the EUV reflectivity of the mask blank 12 will be greater than 70%. A pattern is formed on the mask 10 by forming a layer of non-reflective material 18 on the mask surface, i.e., on the surface of the interference stack 16. The non-reflective material 18 is patterned in a highly accurate manner, e.g., using an electron beam lithography system, to produce a pattern of the non-reflective material 18 on the EUV mask 10 which will define the pattern of the microelectronic or micromechanical structure to be fabricated using the mask 10.
For device fabrication using an EUV mask 12, a photoresist layer deposited on a target substrate wafer to be patterned is positioned to receive EUV radiation that is reflected off of the EUV mask 10. EUV radiation 20, from an EUV source, is directed at the patterned surface of the EUV mask 10. (It is noted that multilayer EUV masks of the type described herein typically are designed for operation at normal incidence of the EUV radiation 20 thereon. A large incidence angle for the EUV radiation 20 is shown in FIG. 1 for ease of illustration.) In areas of the mask surface upon which the patterned EUV non-reflective material 18 remains the impinging EUV rays 20 are absorbed. Hence, there is no reflection of EUV radiation from these areas onto the target substrate. In areas of the EUV mask surface on which there is no remaining non-reflective material 18 the incident EUV rays 20 are reflected 22 and directed to the photoresist covered surface of the target substrate wafer. In this manner, after a development process, the pattern of EUV non-reflective material 18 formed on the surface of the EUV mask 10 is transferred to the target substrate. Up to 20–25 or more masks may be employed for the fabrication of a single integrated circuit chip.
To ensure accurate reproduction of the mask pattern on the target substrate wafer, it is essential both that the pattern of non-reflective material 18 formed on the EUV mask 10 be accurately produced and defect free and that the mask blank 12 on which the EUV mask 10 is formed be defect free. Defects in the mask blank 12, particularly within the interference stack 16, may distort or reduce the intensity of the EUV rays 22 reflected from the mask 10, resulting in a corresponding defect in the pattern formed on the target substrate wafer. Defects as small as 5–10 nm can severely disrupt image formation in EUV lithography for the production of, for example, semiconductor microelectronic circuitry. It is very important, therefore, that any defects in the mask blank 12 be detected, preferably before the mask is patterned for use in fabrication and the resulting error is detected in the end product which the mask is used to fabricate. It also is particularly important that such defects in the mask blank 12 be detected early on, in that, using current technologies, EUV masks can cost $100,000 or more to produce.
Generally, two types of defects in the interference stack 16 of an EUV mask blank 12 typically are encountered, one type may be relatively easy to detect, the other type is much more difficult to detect. The first type of defect is illustrated in FIG. 2. In this case, a defect 24 in the interference stack 16 of the mask blank 12 results in a physical manifestation 26 on the surface of the mask blank 12. This distortion 26 near the surface of the mask blank 12, although small, can adversely affect the reflection of EUV rays from a mask that is made from the mask blank 12, thereby ruining the mask. However, since the defect 24 is manifest as a physical distortion 26 at the surface of the mask blank 12, such a defect may be relatively easy to detect.
A more difficult to detect type of defect that may occur in an EUV mask blank 12 is illustrated in FIG. 3. In this case, the defect 28 is buried in the multilayer interference stack 16. The defect 28 results in a distortion 30 in the layers of the interference stack 16. This distortion will adversely affect the reflection of EUV rays by the interference stack 16, by unpredictably shifting the phase of EUV rays passing through the stack, thereby ruining any mask made from a mask blank having such a defect. In this case, the defect 28 does not manifest itself at the surface of the interference stack 16. Thus, although a defect 28 which can destroy the effectiveness of an EUV mask made from the mask blank 12 is present, the surface of the mask blank may remain perfectly smooth, and may thus appear to be defect free. Such an imbedded defect 28 is, therefore, very difficult to detect. Currently, such a defect 28 could be detected by examining the entire mask blank 12 for defects using a device such as an X-ray microscope, which employs a very narrow penetrating beam to examine the interference stack. However, since defects on the order of 10 nm can ruin the mask blank 12, and a typical mask blank may be four inches by four inches in size, the time required to examine the entire mask blank for such defects using the small field of view provided by an X-ray microscope or similar device makes this method of mask inspection very time consuming, expensive, and, therefore, relatively impractical. At-wavelength inspection of EUV masks also is extremely costly, because of the scarcity of EUV sources.
What is desired, therefore, is a new method for the inspection of EUV lithography masks, and the like, for the detection of defects therein, including defects that are not physically manifested at the surface of the mask blank. The preferred method should be both sensitive, i.e., have the ability to detect very small defects, and accurate, i.e., have the ability to reject background signals. Furthermore, such an inspection method should be able to employ radiation sources other than EUV sources, which may be more readily available.